Bacteria have been isolated which produce metabolites which affect the growth response of E. coli (Davis 113-3) to vitamin B12 (the inhibition is reversible by B12). N5-hydroxy-L-arginine has been identified as one of these compounds. Its effect of E. coli and inhibition of KB cells is potentiated by L-lysine, thialysine, hydroxylysine, and oxolysine.